Currently, optical fibers used for optical fiber communication systems which are generally spreading adopt a structure in which an outer periphery of one core is surrounded by a clad, and information is transmitted when an optical signal propagates in this core. Further, as the optical fiber communication systems spread, the amount of information to be transmitted is dramatically increasing in recent years. Following an increase in the amount of information to be transmitted, the optical fiber communication systems use several tens or several hundreds of multiple optical fibers to perform long-distance optical communication of a large volume.
It is known that, to reduce the number of optical fibers in such an optical fiber communication system, a plurality of signals are transmitted by means of light propagating in respective cores using a multi-core fiber in which outer peripheries of a plurality of cores are surrounded by one clad.
Non-Patent Document 1 below describes such a multi-core fiber. In this multi-core fiber, a plurality of cores are arranged in one clad. For an example of this multi-core fiber, an example is taken in which a single core is disposed in the center of a clad and six cores are disposed around the core disposed in the center. Such a configuration is a structure in which cores can be closely packed, so that a large number of cores can be disposed with respect to the specific outer diameter of the clad. However, as is pointed out in Non-Patent Document 1, in the multi-core fiber, optical signals propagating in respective cores interfere each other, and noise is superimposed on the optical signals propagating in the respective cores in some case. Therefore, Non-Patent Document 1 describes a method for reducing crosstalk in which refractive index differences are changed between the clad and cores adjacent to each other to vary the propagation constants (the wave guide conditions) of the cores adjacent to each other. Moreover, Patent Document 1 below describes a method for varying propagation constants from each other in which refractive index differences or the diameters of cores, for example, are varied from each other. Propagation constants are varied between the cores adjacent to each other as decried above, so that crosstalk can be reduced.
However, even in the multi-core fiber in which the propagation constants of the cores adjacent to each other are different as in Non-Patent Document 1 and Patent Document 1, in the case where the multi-core fiber is bent in a specific bending diameter, one of the cores adjacent to each other is located on the inner side of the arc and the other is located on the outer side of the arc, which sometimes causes the propagation constants of the cores to be matched with each other. Non-Patent Document 2 below describes a multi-core fiber in which an inter-center pitch between cores is set to 30 μm, the MFDs (mode field diameters) of the cores are set to 8.1 μm and 9.4 μm, respectively, and a propagation constant difference is given between the cores. Even in this case, the propagation constants of the cores are matched with each other to degrade crosstalk in the case where the bending radius of the multi-core fiber is about 100 mm.
An optical fiber accommodated in a cable is in a state in which a bend corresponding to a certain radius is applied to the optical fiber. Although the bending radius is greatly different depending on cable designs, the probability of fatigue failure is increased to cause an issue in durability, and bending losses are increased to reduce communication quality at a banding radius smaller than 100 mm. From the viewpoints, such a design is not generally provided in which the bending radius is smaller than a bending radius of 100 mm. Such a bend whose radius is below 100 mm is sometimes applied to input/output ends, branch portions, and the like because of accommodating excessive lengths. However, the lengths are limited as compared with lengths in cable portions, and crosstalk rarely occurs on input/output ends, branch portions, and the like.